LONDON, England (CNN) -- British environmental activists climbed onto the roof of the Houses of Parliament on Wednesday in protest at plans for a third runway at London's Heathrow Airport.

A protester throws a paper aeroplane from the roof of the Houses of Parliament.

Five protesters had gained access to the roof, unfurling a banner reading "No third runway" and throwing paper airplanes, the UK's Press Association reported.

Speaking to CNN via cellphone, one of the protesters, Leo Murray of the "Plane Stupid" campaign group, said it had been "surprisingly easy" to get onto the roof.

"We just went through the front door, then we had a cup of tea, then we took the lift up and we found ourselves on the roof," Murray said.

Murray said Parliament was an appropriate place to protest due to "the little regard this government has shown for the democratic process" amid concerns over the environmental cost of airport expansion.

The protest came on the final day of the government's consultation on plans for Heathrow expansion and was the second major security breach of the week after Greenpeace activists managed to climb on top of a British Airways passenger jet at Heathrow on Monday.

Protester Richard George told PA the case made for the third runway consisted of "gobbledegook and industry spin."

"I am stood on the roof of Parliament because the democratic process had been corrupted. The aviation industry had taken full advantage of a weak Prime Minister to get the Heathrow consultation fixed. It does not even consider global warming despite everything (Prime Minister Gordon) Brown has said about the environment and despite the massive impact aviation has on the climate."

The activists waited until Brown's appearance at the start of Wednesday's Prime Minister's Questions session before ending their protest. They said they would cooperate with police and leave the roof.

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Activists climb onto BA jet at Heathrow

Addressing lawmakers, Brown said: "Decisions in this country should be made in the chamber of this House and not on the roof of this House. It's a very important message that should be sent out to those people protesting."

A prime ministerial spokesman said the incident was a matter for the parliamentary authorities.

Security at Parliament was supposed to have been tightened in recent years following a series of high profile publicity stunts and amid concerns over terrorism.

Visitors pass through airport-style scanners on their way into the building while extra CCTV cameras have been installed. The exterior of the building is guarded by armed police and concrete barricades.

In 2004, pro-fox hunting campaigners, including Otis Ferry, son of rock star Bryan Ferry, gained access to the House of Commons chamber. In the same year, a fathers rights campaigner threw purple powder at then-Prime Minister Tony Blair during Prime Minister's Questions.E-mail to a friend